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Planetary News: Near Earth Objects (2009)Asteroid Tracked in Space, Its Remains Recovered on EarthBy Amir AlexanderMarch 26, 2009
A startling sight greeted the faithful in the Nubian desert of northern Sudan as they emerged from morning prayers on October 7, 2008. White smoky streaks hovered in the air above them, forming ghostly patterns in the morning skies. Faced with this strange phenomenon, the Sudanese villagers did what any citizens of the 21st century would do in such circumstances – they took pictures with their cell-phones. And that is how the impact of asteroid 2008 TC3 – the only asteroid to be tracked in space before striking the Earth – was recorded for posterity. While the Sudanese villagers were undoubtedly surprised by what they saw, scientists throughout the world had predicted the event, and would have given much to be there when it happened. Twenty hours earlier observers at the Catalina Sky Survey near Tuscon, Arizona, were the first to detect an asteroid streaking towards Earth. Other observatories soon joined in tracking the space rock, and when it appeared that it was indeed on a collision course with the Earth, they called Steve Chesley of NASA's Near Earth Object Program at the Jet Propulsion Laboratory in Pasadena. Chesley entered the coordinates into his orbital modeling software, and the answer came back startling and unambiguous: "Impact probability: 100%." It was the first time Chesley had seen such a result, and he rushed to confirm the size and mass of the asteroid. At about 5 meters in diameter and 80 tons, there was little chance that it would reach the surface or cause any damage. The Earth was safe. Thousands of miles away astronomers on La Palma in the Canary islands were also tracking the asteroid. Making use of the William Herschel Telescope they measured the spectrum of light reflected from the asteroid, which provides essential information about its composition. It was a difficult feat of observation, with the asteroid so close and moving so fast across the sky, but the La Palma team proved up to the challenge. By the time they were done only four hours remained before impact, and with 1 hour to go the asteroid disappear beyond the horizon of the tracking observatories. The next people to see it were the Sudanese residents of the Nubian desert, as they exited their mosques on the morning of October 7.
2008 TC3, as the asteroid was designated, did not survive its entry into the atmosphere. At a height of 37 kilometers it exploded, and its tiny fragments were consumed in the atmosphere, producing the vapor trails that were recorded on the local residents' cell phones. As far as most astronomers were concerned, that was the end of the story. But Peter Jenniskens, a meteor astronomer at NASA Ames and the SETI Institute, wasn't so sure. Several weeks after the event he contacted Chesley, and asked for the precise coordinates of the asteroid's impact, based on its trajectory in space. He then teamed up with Mauwia Shaddad, a physicist at the University of Khartoum in Sudan, to organize an expedition to search for any remaining fragments from the asteroid. On December 5, 2008, Jenniskens, Shaddad, and 45 students and staff set from "Station 6" of the local railway line along the general flight trajectory of 2008 TC3. Because the asteroid had exploded at such a great height, they expected that any remaining meteorites would be scattered over a large area, making them particularly difficult to locate. But only two hours into the search one of the students, Mohammed Alameen, found a black odd-looking rock that looked like a meteorite. It proved to be the first fragment of 2008 TC3, now named "Almahata Sitta," Arabic for "Station 6." As the expedition moved along the asteroid's path they recovered larger and larger pieces. In repeated searches over several weeks they collected nearly 280 pieces of the asteroid strewn over 29 kilometers of desert. Altogether the meteorites weighed less than 5 kilograms – all that remained of the 80 ton asteroid. It was a remarkable find: For the first time ever a celestial body that was observed in space was subsequently collected on the ground and made available for laboratory study. The measurements taken by the Herschel Telescope in La Palma were now compared with the direct analysis of the meteorite shards. The two were found to be in close agreement, providing an excellent test of the reliability of spectral analysis. The close correspondence also suggested that small bodies like 2008 TC3 are not covered with a heavy coating of dust, which can distort the spectroscopic measurements for larger objects.
Almahata Sitta, it turned out, is an extremely rare type of space rock, known as an F-class "ureilite." A close analysis showed that it is extremely porous and fragile, which is why it had exploded at such a high altitude. All ureilites are thought to have originated from a single primordial source, and the structure of Almahata Sitta indicates that it was a volcanically active body in which gas bubbles were trapped inside the porous rock. "The recovered meteorites," said Jenniskens, "were unlike anything in our meteorite collections." "It was," said Lucy McFadden, professor of astronomy at the University of Maryland, "a very exciting result." The tracking and recovery of Almahata Sitta was also significant for the protection of the Earth from future impacts. If at some time in the future it appears that our planet could be struck by a much larger F-class ureilite, scientists can draw on their close analysis of the porous meteorite to evaluate the threat. Most importantly, however, astronomers have shown that they can observe a Near Earth Object in space before it strikes the Earth, and predict the place of impact with remarkable accuracy. "This event," said Alan Fitzsimmons of Queen's University in Belfast, shows we can successfully predict the impact of asteroids, even with a short warning time."
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